1. Field of the Invention
The present invention relates to a process for reclaiming a wafer substrate from a semiconductor wafer, and more particularly to a process for manufacturing a reclaimed wafer that has substantially the same quality as prime wafers used for the manufacture of semiconductor circuit parts. Especially, the process of the present invention reclaims used wafers with a low metallic contamination level.
2. Description of the Related Art
Single-crystal silicon wafers having two different qualities are used in semiconductor manufacturing processes. xe2x80x9cPrimexe2x80x9d quality wafers are used for manufacturing actual semiconductor products, and xe2x80x9ctestxe2x80x9d quality wafers are used for confirming whether the manufacturing processes have satisfactory performance. xe2x80x9cPrimexe2x80x9d wafers satisfy higher quality standards than xe2x80x9ctestxe2x80x9d wafers. xe2x80x9cTestxe2x80x9d wafers having quality comparable to that of xe2x80x9cprimexe2x80x9d wafers are preferred by semiconductor companies and are sold at a higher price than standard quality xe2x80x9ctestxe2x80x9d wafers. A typical used semiconductor wafer comprises a silicon wafer with semiconductor components implanted and/or diffused into one wafer surface (hereinafter called xe2x80x9cactive surfacexe2x80x9d). In addition, coating layers of conducting and insulating materials are formed on the implanted and/or diffused surface of the wafer.
The term xe2x80x9csurface layersxe2x80x9d as used herein refers to both implanted and/or diffused portions of an original wafer and layers formed or deposited on the surface of the original wafer.
Reclaiming implies removing both the surface layers and subsurface portions of the wafer just underlying the implanted and/or diffused portions at the wafer surface. Used wafers sent to a reclamation service company have a variety of surface and subsurface structures. Some wafers have been used for film thickness measurement and have several film layers on the surface. Others may be rejects from the product wafer manufacturing processes and have layered structures, film material combinations and implanted elements which differ from one wafer to another. The reclaimed wafer is required to have surface properties similar to those of a new wafer and a metallic contamination level at an order as low as that of a new wafer.
Heretofore, several methods have been employed to reclaim a wafer substrate from a used semiconductor wafer.
Chemical etching is the most common technique in the conventional reclamation methods. For example, Japanese Unexamined Patent Application Publication No. 51-1966 discloses a process of immersing a wafer substrate having surface layers in a bath that contains sulfuric acid for removing organic-materials, hydrochloric acid and nitric acid for removing metallic materials, and hydrofluoric acid for removing oxides and nitrides. It is further disclosed that a preferable etching rate is 12 xcexcm per minute. Also, a mixture of nitric acid (HNO3) and hydrofluoric acid (HF), which is capable of removing most of films and implanted elements, is often used in etching to reclaim silicon wafers. After thus removing the surface layers by chemical etching, a front surface of a wafer is finished into a mirror surface by chemical mechanical polishing.
U.S. Pat. No. 5,131,979 discloses a wafer reclaiming process featured in comprising stripping away surface layers to expose the original wafer surface by chemical etching, alone, or mechanical grinding followed by chemical etching. After removing the wafer edge, the surface is polished smooth and thermally treated to free the surface of lattice defects and to leave in the interior lattice defects that will become gettering sites.
In the case of removing surface layers of a wafer substrate by chemical etching, materials making up the surface layers are gradually dissolved in a chemical etchant. Since the surface layers contain a metallic film, e.g., an aluminum film, in addition to an insulating coating such as silicon oxide, the content of a metallic component dissolved in the chemical etchant increases with the progress of the reclamation process. As the content of the metallic component in the chemical etchant increases, an amount of the metallic component attaching again to surfaces of the wafer, which is immersed in the chemical etchant for removing the surface layers, also increases correspondingly. The metallic component attaching again to a front surface of the wafer substrate can be avoided from remaining still in a final product, because the substrate front surface is removed by a subsequent step of chemical mechanical polishing. However, a back surface of the wafer substrate remains the same in the final product as just after being etched by the chemical etchant. Accordingly, if there occurs metallic contamination on the back surface, the final product cannot satisfy the quality requirements regarding metallic contamination. As shown in the road map laid open by the SIA, in particular, a metallic contamination level required for semiconductor wafers tends to become strict more and more in the future. The technique for avoiding metallic contamination is therefore essential, taking into account a limitation in management of a metallic contamination level of the chemical etchant used for removing the surface layers.
Further, scratches often occur in surfaces of a used wafer due to rough handling or contact with equipment in a semiconductor company. A chemical etching rate has a tendency to increase in an area where a mechanical damage such as a scratch is present. Accordingly, the scratch is rather intensified by chemical etching in some cases. If a scratch is left on the back surface, the scratch remains as it is in a final product, and therefore the final product cannot satisfy the quality requirements with respect to the back surface.
Moreover, stains attributable to residuals of chemical fluids or deposition of reaction products are produced on the wafer surfaces after the chemical etching. If such stains are produced on the back surface, the stains remain as they are in a final product, and therefore the final product cannot satisfy the quality requirements with respect to the back surface.
An object of the present invention is to provide a process capable of reclaiming used semiconductor wafers with a reduced metallic contamination level on wafer surfaces.
To achieve the above object, according to the present invention, a process for reclaiming a used wafer substrate comprises the steps of removing one or more surface layers of the substrate by chemical etching; scraping off one surface of the substrate in a small amount by blasting; removing a damage layer, which has occurred due to the blasting, by chemical etching; and polishing the other surface of the substrate into a mirror finish.